This invention relates generally to a system for the removal of filtered particles collected on porous filter material of a particle filter device adapted for gaseous media flow, particularly for the exhaust flow of a diesel engine to filter particles from the exhaust gases discharged from the engine, the removal being effected by oxidation.
It is known that in order to reduce particle emission in diesel engines, after-treatment systems are mounted in or operatively associated with the engine exhaust system. Such after-treatment systems essentially comprise filtration systems which trap and collect the solid and vaporous portions of the particle phase. The particles deposited in the filter, however, effect an increase of the flow resistance in the exhaust system, causing an increase in the exhaust back-pressure of the engine. With an increase in particle quantity, such can lead to engine stalling, depending on load and rpms. For this reason, it is requisite that the particles deposited in the filter be removed, either continuously or intermittently. And, it is usual for this removal to be carried out by means of oxidation of the particles.
Ceramic filters with a honeycomb structure, steel-wool filters and ceramic foam with or without a catalytic coating are among the structures which have been employed as filtration systems for the collection of particles, with intermittent or continuous particle incineration.
In order to initiate soot or particle incineration in the filter, sufficiently high temperatures and oxygen content are required. In this regard, this so-called minimum regeneration temperature depends on the mass of each particle collected in the filter, on the oxygen content of the exhaust gas, as well as on the mass flow of particles carried off and deposited in the filter. For production auto diesel engines at a filter efficiency level for solid components of approximately 90%, the lowest regeneration temperature without additional measures amounts to approximately 500.degree. C., whereby the oxygen content must be greater than 3%.
Since without additional measures, these high temperatures can only be reached, in the exhaust of a diesel engine, in the full load range, though a filter regeneration is nonetheless required even at lower temperatures, several measures can be considered. With the provision of engine-related measures such as a injection timing and intake-air choking, as well as exhaust back-pressure increase, the exhaust gas temperature can be increased to regeneration in the area of low loads and rpms. By means of catalytic coating of the filter, a reduction of the regeneration temperature can be achieved. In addition, there is the possibility of achieving a reduction of the soot ignition temperature and regeneration by means of additives to the fuel. Such known measures are described, for example, in U.S. Pat. No. 4,462,812, column 6, lines 44-62.
Research has shown that in initiation of regeneration, soot oxidation begins at one location in the particle filter (the ignition core), and continues by spreading from this location in both axial and radial directions. In those instances where the filter is freely impacted by the inflow, such that at first approximation a largely homogeneous distribution of particles in the filter takes place, it has been shown, particularly in those instances where additives to the fuel have been employed as an aid to regeneration, that the location of the ignition core in the particle filter is spacially stochastically distributed. Moreover, the difficulty in the use of additives is that such can permit soot ignition to be effected at exhaust temperatures of well under 500.degree. C., such as in the range of 100.degree.-100.degree. C. At such exhaust temperatures, the engine operates in engine map ranges of low load and rpms, but at a high oxygen content of the exhaust.
The soot combustion proceeds in radial and axial directions from the ignition core, whereby, especially at low loads and rpms, the oxidation occurs preferentially in the axial direction, and the soot combustion thereupon continues radially to the ignition channel. The sequential combustion-freeing of areas of the filter is such that in those areas of the filter where the soot has been burned up, flow resistance is considerably lower than in those areas where soot deposits remain. As a result, the larger portion of the exhaust flows through the filter areas with low resistance (regenerated filter areas), causing those areas where the soot remains deposited and the soot oxidation is taking place to experience a slight flow-through of exhaust gas. Since soot oxidation is an exothermic process, heat is released in this process, which is then carried away by the exhaust gas flow.
Due to a non-homogeneous distribution of the exhaust gas mass flow in partially regenerated filters, the heat removal in those areas of the filter which are burned free toward the end of the regeneration process, is very slight. This causes high temperatures to build up in the burning layer, and hence also in the filter material. The result is a very strongly non-homogeneous temperature in the filter with cool areas (regenerated filter portions with filter material temperatures equal to the exhaust gas temperature) and filter areas in the process of regeneration, which have slight heat removal from the exhaust gas, and in which peak temperatures occur. These high temperature levels lead to thermal stress, which often cause the destruction of the filter. In addition, with soot oxidation at slight removal of heat by the exhaust gas, high temperatures occur in the wall areas, so that the filter material (e.g., ceramic material with a melting temperature of 1350.degree. C.) melts. Chipped filters and filters with melting of the material in the channels lead to a severe reduction of filter efficiency. Thus, with a honeycomb-shaped filter body with 100 cells/square inch and a diameter of 4.66 inches in which 3-4 channels are defective, a reduction of the level of filter efficiency of aproximately 30% is found.